The present invention relates a current monitoring system and, more particularly, to a branch circuit monitor configurable to enable current transformers with a plurality output voltage ranges to be used with the branch circuit monitor.
The total power consumption of a building or other facility is typically monitored by the electric utility with a power meter located between the utility's distribution transformer and the facility's power distribution panel. However, in many instances it is desirable to sub-meter or attribute the facility's power usage and cost to different occupancies, buildings, departments, or cost centers within the facility or to monitor the power consumption of individual loads or groups of loads, such as motors, lighting, heating units, cooling units, etc. These single phase or multi-phase electrical loads are typically connected to one or more of the branch circuits that extend from the facility's power distribution panel. While a power meter may be installed at any location between a load and the distribution panel, typically a power meter capable of monitoring a plurality of circuits is installed proximate the power distribution panel to provide centralized monitoring of the various loads powered from the panel.
Flexibility has favored adoption of digital current and power meters, known as branch circuit monitors, incorporating data processing systems that can monitor a plurality of circuits and determine a number of parameters related to electricity consumption. A digital power meter for measuring electricity consumption by respective branch circuits comprises a plurality of voltage and current transducers that are periodically read by the meter's data processing unit which, in a typical digital power meter, comprises one or more microprocessors or digital signal processors (DSP). The data processing unit periodically reads and stores the outputs of the transducers sensing the magnitudes of the current and voltage and, using this sample data, calculates the current, voltage, power, and other electrical parameters, such as active power, apparent power and reactive power that quantify electricity distribution and consumption. The calculated parameters are typically output to a display for immediate viewing and/or transmitted from the meter's communications interface to another data processing system, such as a building management computer for remote display or further processing, for example formulating instructions to automated building equipment.
While the voltage transducer(s) of a digital power meter can be interconnected anywhere in the wiring that connects the supply and a load, including at the load's terminals, interconnection of voltage transducers and the facility's wiring is facilitated by wiring connections in the power distribution panel. The power distribution panel provides a convenient location for connecting the voltage transducers because typically each phase of the power is conducted by a separate bus bar in the power distribution panel and the voltage and phase is the same for all loads attached to the respective bus bar. The voltage transducers of digital power meters commonly comprise a voltage divider network that is connected to a conductor in which the voltage will be measured.
The current transducers of digital power meters typically comprise current transformers that encircle each of the power cables that interconnect each branch circuit to the bus bar(s) of the distribution panel. A current transformer typically comprises multiple turns of wire wrapped around the cross-section of a toroidal core. The power cable, conducting the load current, is passed through the aperture in the center of the toroidal core and constitutes the primary winding of the transformer and the wire wrapped around the cross-section of the core comprises the secondary winding of the transformer. Current flowing in the primary winding (primary current) induces a secondary voltage and current in the secondary winding which is quantitatively related to the current in the primary winding. The measurable output of a current transformer can be the current in the secondary winding or the voltage across a burden resistor connected in parallel with the secondary winding of the transformer. Output voltage ranges for current transformers are typically 0-0.333 volts (v.), 0-1 v., 0-5 v. or 0-10 v. However, current transformers with other output voltage ranges are available.
Typically, branch circuit monitors are designed for use with a particular current transformer having a single output voltage range. However, a user may desire to use a different transformer than the one specified for use with a particular meter. For example, when replacing a meter, there can be significant savings by using the current transformers for the previous meter. What is desired, therefore, is a branch circuit monitor providing flexible construction, simplified installation and improved serviceability.